The invention relates to a method of scanning original images.
Charged coupled device (CCD) sensors are regularly used in image scanning equipment for sensing the colour content of an image. This information is then digitized and can be manipulated and/or stored for subsequent output or transmission. Most colour input scanners presently on the market use a 3-stripe CCD image sensor as a means of sensing the original image. This device has three lines of photosensitive elements (photosites) onto which the original is imaged. Each line has a different colour filter (usually red, green and blue) deposited in front so that the device becomes colour sensitive. The CCD is moved relative to the original during a scan so that a 2-D image of the original, separated into three colours, is built up.
When scanning continuous tone colour graphics such as colour transparencies or colour negatives, the CCD needs certain requirements to achieve the best performance:
1) The size of the photosites determines how many electrons are stored in each site during an exposure period. The larger the photosites, the more electrons can be stored and therefore the dynamic range of the device is increased i.e. the range of light intensity in the original image over which detail can be seen before the device saturates at the high end or detail gets lost in the noise at the low end is increased.
2) Since the output from each photosite represents a discrete sample of the original image light intensity, then Lo achieve the best sharpness in the scanned image, a large number of photosites is required. However, since the original film image is made from colour dye grains, there is no benefit to be gained from resolving detail much smaller than the grain size.
Therefore, a good compromise can be had between maximising dynamic range and achieving the best sharpness before film xe2x80x9cGrain noisexe2x80x9d is seen. Presently available CCDs typically use 8000 photosites per line with a dynamic range of about 80 dB i.e. a range of about four density in light intensity terms or a greater number of smaller photosites e.g. between 10,000 and 15,000 with less dynamic range.
Many scanners need to be multi-purpose and need to be able to scan other forms of original e.g. lineart (linework) or screened film originals (for the purpose of descreening or copydot). Lineart typically consists of solid black lines on a white background although any colour on any other colour background can be used instead. Typical examples of lineart are line drawings or solid text or a combination of both. The detail present, especially in very fine Kanji text, can be very high. Screened film originals consist of solid black dots of varying sizes on a white background and again there is a high level of detail present in this type of original.
When scanning lineart or screened film originals, the CCD needs a different set of requirements to achieve the best performance.
1) The scanner now has only to be able to set a threshold to differentiate between black and white and therefore the dynamic range of the CCD does not need to be so high and therefore the size of the photosites can be less.
2) Since the CCD needs to be able to resolve finer detail, the more photosites that can be placed across an image of the original the better. To achieve an optical resolution of 1200 photosites per inch (often quoted as 1200 dpi) across the narrow width of an A3 original requires at least 14,000 photosites and allowing for some overscan of the original, at least 16,000 photosites is desirable.
3) Since lineart is not restricted to black on white and can be any colour, it is preferable not to have colour filters on the CCD so that separation of colours can be performed using external filters.
Presently available 3-stripe colour CCDs cannot achieve the best performance when scanning all these types of originals.
WO-A-91/00667 describes the use of a 4-stripe CCD with the 4th line (monochrome) having a higher resolution than the other three colour lines. This is used in a motion picture film scanner application for increasing the detail in the picture. The three colour lines provide low definition colour components and the 4th high resolution mono line is added in to provide detail enhancement. This is therefore does not address lineart scanning.
JP-A-62161255 also discloses a 4-stripe CCD but used in a photocopying machine. In this patent, a 4-stripe COD is used which consists of a 3-stripe CCD (array 1) and a high resolution stripe (array 2) with no colour filter. Since the application is for a photocopier, there are no prescan or HR scan states. The operator, via the console, stares whether the image is graphics or lineart. If graphics is selected, then the whole image is scanned at graphics resolution using array 1 and output to the print engine. If lineart is selected, then the whole image is scanned at a higher resolution using array 2 and output to the print engine. The system cannot accurately reproduce an original image having both graphics and lineart.
In accordance with one aspect of the present invention, a method of scanning an original image containing image information of different types comprises causing relative scanning movement between a CCD sensor and the original image while the original image is illuminated, the CCD sensor including on the same substrate at least one first linear array of photosites, and a second linear array of photosites, the pitch between adjacent photosites of the second linear array being different from the pitch between adjacent photosites of the or each first array; monitoring output signals from the at least one first linear array when the at least one first linear array is scanned over a first type of image information; and monitoring output signals from the second linear array when the second linear array is scanned over a second type of image information.
In accordance with a second aspect of the present invention a scanning system comprises a CCD sensor including on the same substrate at least one first linear array of photosites, and a second linear array of photosites, the pitch between adjacent photosites of the second linear array being different from the pitch between adjacent photosites of the or each first array; an image support; means for irradiating an original image on the support; means for causing relative scanning movement between the image support and the CCD sensor whereby radiation from the original image impinges on the CCD sensor; and a processing circuit coupled to the CCD sensor for regularly downloading image information from the CCD sensor and for carrying out a method according to the first aspect of the invention.
With this invention, we use the second linear array of photosites which has a different resolution from the other array(s) of photosites to enable a second type of information to be scanned at the same time as the first type of information. Thus, an original image having more than one type of information can be scanned in a single pass rather than two passes.
In a simple application, the monitored output signals can be displayed but typically they will be stored for further processing.
In one application, output signals from the at least one first linear array will be monitored only when that array is scanned over a first type of image information and output signals from the second linear array will be monitored only when that array is scanned over the second type of image information. Alternatively, output signals from both arrays can be monitored throughout the scanning process and then subsequently processed so that a representation of the original image is generated in which data corresponding to output signals from the at least one first linear array is used to define pixels corresponding to the first type of image information, and data corresponding to output signals from the second linear array is used to define pixels corresponding to the second type of image information.
The location on the original image of different types of information can be defined by the operator manually. In a first alternative, however, the method may further comprise performing a prescan of the original image to define the locations of image information of the different types.
In a second alternative, the type of information being scanned could be determined automatically using, for example, the method described in more detail in EP-A-0881595 incorporated herein by reference.
The xe2x80x9coriginal imagexe2x80x9d may comprise a single page, for example an A4 sheet, or multiple pages, for example as mounted on the drum of an image scanner or platen of a flat bed scanner.
A variety of different types of information can be scanned. Typically, the first type of information will comprise graphics while the second type of information may comprise lineart or screened images.
Typically, the photosite pitch of the second linear array will be smaller than the pitch of the photosites of the first array so that it can be used for scanning lineart or screened film originals. Thus, the or each first linear array can be fabricated with photosites which are made as large as possible with enough photosites to be able to resolve detail comparable to the film grain size. In this way, the dynamic range is maximised and the sharpness in the scanned image is maximised before film dye grain noise degrades the image. A typical pitch of the photosites of the or each first linear array is in the range 5-12 microns, while the photosite shapes could be rectangular or square. A typical number of photosites is in the range 5000-15000 per line.
In the preferred example, the second linear array has a larger number of smaller photosites so that extra detail can be resolved in the scanned image at the expense of a loss of dynamic range. A typical pitch for the photosites of the second linear array is 2-5 microns and a typical number of photosites for the line is in the range 10000-30000.
In a very simple example, a single first linear array could be provided along with a single second linear array. However, more than one first linear array could be provided and typically there are three, one corresponding to each of the primary colours red, green and blue.
The linear arrays are typically single, linear arrays but in some cases they could each form part of a respective two-dimensional array.
The wavelength sensitivity of each array is typically determined by the use of suitable internal or external filters. Thus, as in conventional CCD arrays, the or each first linear array may have a suitable filter material deposited on it or radiation reaching the first array may have to pass through a suitable external colour filter.
In general, the second linear array will have no colour filter so that lineart of any colour can be scanned. However, a coloured filter could be deposited on the second linear array for the purpose of restricting the range of wavelengths that the lens in a scanner with which the CCD sensor is used sees so that the best sharpness performance of the lens is realised i.e. loss of sharpness due to chromatic abberation is minimized. The second linear array could also have a colour filter deposited for the purpose of restricting the range of wavelengths so that when scanning screened film originals, the visibility of Newton""s rings between the original and the platen glass is minimized.